1. Field of the Invention
The present invention relates to a process for producing a compound semiconductor single crystal. Further, the present invention relates to a useful technique which is adapted to, for example, the vertical gradient freezing method and the vertical Bridgman method in which a single crystal is grown in a vertical direction by cooling a raw material melt of a compound semiconductor.
2. Description of the Related Art
In general, when a compound semiconductor single crystal is produced, the liquid encapsulated Czochralski method (hereinafter, referred to as “LEC method”), or the horizontal Bridgman method (hereinafter, referred to as “HB method”) is industrially used. In the LEC method, there are some merits, for example, that a wafer having a cross section formed in a circle and a large diameter can be obtained, and that a crystal having a high purity can be obtained by using a liquid encapsulant (B2O3). However, there are some demerits, for example, that because a temperature gradient in the crystal growth direction is high, a dislocation density in the crystal becomes high. As a result, electric properties of an electronic device, such as an FET (field effect transistor) or the like, which is made from the obtained crystal, deteriorate.
On the other hand, in the HB method, there is a merit that because a temperature gradient in the crystal growth direction is low, a crystal having a low dislocation density can be obtained. However, there are some demerits, for example, that because a raw material melt of a compound semiconductor is solidified in a crucible formed in a boat-shape, it is hard to enlarge the diameter of the obtained wafer. Further, the obtained wafer has a cross section formed in a substantial D-shape.
As a crystal growth method having the merits of the LEC method and those of the HB method, the vertical gradient freezing method (hereinafter, referred to as “VGF method”) and the vertical Bridgman method (hereinafter, referred to as “VB method”) have been proposed. The VGF method and the VB method are carried out as follows. A crucible containing a compound semiconductor raw material is disposed in a vertical type of heating furnace. After the raw material is heated and molten by a heater, the raw material melt is gradually solidified in the vertical direction from a seed crystal portion disposed on the bottom portion of the crucible to grow a single crystal. In these methods, because a cylindrical crucible is used, a circular wafer can be obtained. Further, because a temperature gradient in the crystal growth direction is low, a crystal having a low dislocation density can be obtained.
The liquid encapsulated Kyropoulos method (hereinafter, referred to as “LEK method”) in which a seed crystal disposed in an upper portion of a crucible containing a raw material is in contact with a surface of the raw material melt and then a single crystal is grown in the crucible by gradually solidifying the melt from the surface thereof, have a merit that a circular wafer can be obtained and that a crystal having a low dislocation density can be easily obtained, like the VGF method and the VB method.
However, a crystal grown in the VGF method, the VB method or the LEK method, has a demerit that a twin or a polycrystal is easily generated from a seed crystal to a body part of the crystal. Further, the yield of the single crystal is low. In particular, because a twin or a polycrystal is easily generated in a material having a low stacking fault energy or a low critical shear stress, it is hard to grow a single crystal of the material.
Further, in the VGF method or the VB method, because a crystal is grown by disposing a seed crystal on the bottom portion of the crucible, it is necessary to use the crucible having a containing portion for a seed crystal on the bottom thereof. However, such a crucible has a demerit that because the crucible has a special shape, it is more expensive than a crucible having a flat bottom, which is generally used. Further, the seed crystal containing portion of the crucible is easily damaged. Therefore, the producing cost of the crystal increases.
The inventors have studied a process for growing a crystal on the basis of a nucleus generated by adjusting a temperature of a raw material melt and a pressure thereof without a seed crystal in the VGF method, the VB method or the LEK method. The process was an effective one for growing a group II-VI compound semiconductor single crystal in which a twin or a polycrystal was easily generated from a seed crystal to a body part of the crystal. Further, in the process, because a crucible having a flat bottom might be used, the used crucible was relatively cheap and was not easily damaged. Further, because it was not necessary that a seed crystal was used, the cost of the seed crystal was not required. The process had the advantage of reducing the producing cost of the crystal.
In the process for crystallizing a raw material melt without a seed crystal as described above, it is necessary that a nucleus is reproducibly generated in a raw material melt. However, when the raw material melt is gently and slowly cooled in a state that a seed of the crystal does not exist in the raw material melt, there is some possibility that a supercooling state in which the melt is not transited to solid even though the temperature of the melt is not higher than the melting point thereof, is caused. In this case, a plurality of nuclei are generated when the temperature of the melt is several tens ° C. lower than the melting point thereof. Because the melt is rapidly solidified on the basis of the nuclei, a polycrystal or a twin is grown. As a result, a single crystal is not grown.
As described above, in the process for growing a crystal without a seed crystal, the possibility that the supercooling state is caused becomes high according to a material of a crystal to be grown and a growing condition. As a result, there is a problem that the percentage of growing the single crystal is low.
According to the report (M. Muhlberg et al. Journal of Crystal Growth, Vol. 128, (1993) pp 571–575), when a deviation between a temperature at which a supercooled liquid is solidified and the melting point of the supercooled liquid is defined as a degree of supercooling, the degree of supercooling is correlated with a holding temperature of the melt. In the concrete, it is reported that the degree of supercooling becomes low when a difference between a temperature at which the raw material is held in a melt state and the melting point of the raw material is small.
The inventors noticed the study described in the above report and studied the relationship between the holding temperature of the melt and the occurrence of the supercooling state by using a compound ZnTe. As a result, the inventors found that the melt was not in the supercooling state and the melt begun to be solidified near the melting point thereof by holding the melt in the temperature range of the melting point to (the melting point+8)° C.
According to this finding, when a crystal is grown by gradually cooling the raw material melt after the raw material melt is held in the above temperature range, the supercooling state is not caused. Therefore, a single crystal ought to be obtained. However, although the crystal growth was carried out by the above process ten and several times, the possibility that a single crystal could not be obtained was about 50%. A single crystal could not be reproducibly grown only by adjusting the condition of the holding temperature of the melt.
The inventors studied the cause that the single crystal could not be obtained. As a result, the inventors found that the crystal which was obtained by generating a plurality of nuclei on the surface of the melt and then growing a ZnTe crystal around each nucleus, was a polycrystal. Further, a part of raw material kept unmolten when the holding temperature of the melt was near the melting point. It was found that the ZnTe polycrystal was grown on the surface of the melt by the unmolten material.
In order not to leave the unmolten material on the surface of the melt, the inventors studied the holding time of the melt and the holding temperature thereof. However, although the holding time of the melt was extended, the effect that the unmolten material was not left could not be obtained remarkably. When the holding temperature of the melt increased, the possibility that the supercooling state was caused became high. As a result, a polycrystal or a twin was generated. Therefore, these solutions could not be effective.